Price elasticity of demand measures how responsive quantity demanded is to a change in price, expressed as a ratio of percentage changes. It's one of the most practical tools in microeconomics because it connects abstract demand theory to concrete pricing and revenue questions.

The basic formula is:

$E_d = \frac{\%\Delta Q_d}{\%\Delta P}$

Because demand curves slope downward (price and quantity move in opposite directions), this ratio is naturally negative. In practice, you'll typically work with the absolute value so you can classify elasticity by magnitude without worrying about the sign.

There are two main ways to compute elasticity depending on what information you have:

Arc (midpoint) elasticity works when you have two discrete price-quantity points. It uses the average of the initial and final values as the base, which eliminates the asymmetry problem you'd get if you computed percentage changes from just one endpoint:

$E_d = \frac{(Q_2 - Q_1)\,/\,(Q_2 + Q_1)}{(P_2 - P_1)\,/\,(P_2 + P_1)}$

Point elasticity works when you have a continuous demand function and want elasticity at a specific point. You take the derivative of quantity with respect to price and scale it:

$E_d = \frac{dQ}{dP} \cdot \frac{P}{Q}$

Use point elasticity when you're given a demand equation (e.g., $Q = 100 - 2P$ ). Use arc elasticity when you're given a table of prices and quantities or two observed data points.

A common mistake: don't confuse the slope of the demand curve with elasticity. Slope is $\Delta Q / \Delta P$ , a constant along a linear demand curve. Elasticity changes along that same curve because the $P/Q$ ratio changes at every point.

Interpreting Elasticity Values

Elasticity Categories

The absolute value of $E_d$ tells you which category demand falls into:

| Category | $|E_d|$ | What It Means | Typical Examples | |---|---|---|---| | Elastic | > 1 | Quantity responds more than proportionally to price | Restaurant meals, airline tickets | | Inelastic | < 1 | Quantity responds less than proportionally to price | Gasoline, insulin | | Unit elastic | = 1 | Quantity and price change by the same percentage | Theoretical benchmark | | Perfectly elastic | $\infty$ | Any price increase kills all demand (horizontal curve) | Individual firm in perfect competition | |Perfectly inelastic|0|Quantity doesn't change at all (vertical curve)|Life-saving medication with no substitute| For a linear demand curve $Q = a - bP$ , elasticity varies from 0 (at the quantity-axis intercept) to $-\infty$ (at the price-axis intercept), passing through unit elasticity at the midpoint.

Defining and Calculating Elasticity, Elasticity | Microeconomics

Elasticity and Total Revenue

The total revenue test is a quick way to figure out whether demand is elastic or inelastic without computing the exact number. Total revenue is $TR = P \times Q$ , so what happens to $TR$ when price changes depends on which effect dominates:

Elastic demand ( $|E_d| > 1$ ): A price decrease raises total revenue. The percentage gain in quantity more than offsets the percentage loss from a lower price.
Inelastic demand ( $|E_d| < 1$ ): A price increase raises total revenue. Quantity barely drops, so the higher price per unit wins out.
Unit elastic demand ( $|E_d| = 1$ ): Total revenue stays constant. The two effects exactly cancel.

This relationship is formalized by the marginal revenue equation for a firm facing a downward-sloping demand curve:

$MR = P\left(1 + \frac{1}{E_d}\right)$

When $|E_d| > 1$ , marginal revenue is positive (selling more adds to revenue). When $|E_d| < 1$ , marginal revenue is negative (selling more actually reduces revenue). A revenue-maximizing firm never operates on the inelastic portion of its demand curve.

Factors Influencing Elasticity

Product Characteristics

Several features of a good determine how elastic its demand will be:

Availability of close substitutes is the single biggest factor. Coke vs. Pepsi are near-perfect substitutes, so a small price increase on Coke sends many buyers to Pepsi. But "soft drinks" as a broad category has fewer substitutes, making demand more inelastic. The narrower you define the product, the more elastic demand tends to be.
Share of the consumer's budget matters because people pay closer attention to price changes on expensive items. A 10% increase in the price of housing hits your budget hard; a 10% increase in the price of salt is barely noticeable.
Necessity vs. luxury shapes responsiveness. Demand for basic groceries is inelastic because you need to eat regardless of price. Demand for vacation travel is elastic because you can postpone or skip it.
Habit-forming or addictive goods (cigarettes, coffee) tend to have inelastic demand. Physiological or psychological dependence makes consumers less willing to cut back when prices rise. Empirical estimates put the price elasticity of cigarettes around $-0.4$ , meaning a 10% price hike reduces consumption by only about 4%.

Defining and Calculating Elasticity, Elasticity – Introduction to Microeconomics

Market and Time Factors

Time horizon is critical. In the short run, consumers are stuck with existing habits, contracts, and durable goods. Over time, they find substitutes, change habits, or switch technologies. Gasoline demand is quite inelastic in the short run (you still need to drive to work) but more elastic over several years (you can buy a fuel-efficient car, move closer to work, or switch to public transit).
Consumer information affects responsiveness. When buyers can easily compare prices across sellers, demand for any single seller's product becomes more elastic. This is why online retail markets tend to exhibit higher elasticities than brick-and-mortar stores for the same goods.
Market structure plays a role at the firm level. A firm in a perfectly competitive market faces perfectly elastic demand for its output (it's a price taker). A monopolist faces the market demand curve, which is less elastic.

Elasticity in Pricing Decisions

Revenue Optimization and Price Discrimination

Firms with market power use elasticity estimates directly in pricing:

Identify the elasticity of each customer segment. Business travelers have inelastic demand for flights (they need to be somewhere on a specific date). Leisure travelers have elastic demand (they can be flexible on dates or destinations).
Set higher prices for inelastic segments and lower prices for elastic segments. Airlines charge business class fares several times higher than economy precisely because business travelers are less price-sensitive.
Adjust prices over time using the total revenue relationship. If raising the price on a product increases revenue, demand is inelastic at the current price. If it decreases revenue, demand is elastic, and the firm should consider lowering the price instead.

This logic extends to tax incidence as well. When a government imposes a per-unit tax, the burden falls more heavily on the side of the market (buyers or sellers) that is more inelastic. If demand is very inelastic relative to supply, consumers bear most of the tax.

Industry Applications

Utilities use peak-load pricing because demand elasticity varies by time of day. During peak hours, demand is more inelastic (people need electricity right then), so higher rates both raise revenue and manage load.
Digital platforms exploit elasticity variation through freemium models. The free tier targets highly elastic users who would never pay; the premium tier captures inelastic users who value additional features enough to pay.
Cross-price elasticity ( $E_{xy} = \frac{\%\Delta Q_x}{\%\Delta P_y}$ ) helps firms anticipate how a price change on one product affects demand for another. A positive cross-price elasticity means the goods are substitutes; a negative value means they're complements. A coffee shop raising its coffee price should expect a drop in pastry sales if the two are complements.
Oligopolistic firms pay special attention to elasticity because rival reactions amplify or dampen the effects of their own price changes. The kinked demand curve model, for instance, assumes demand is more elastic for price increases (rivals don't follow) than for price decreases (rivals match), discouraging price changes in either direction.